An electromagnetic wave, such as a light wave, has an associated vector field that has two primary and orthogonal polarization states or vector directions. When referring to the propagation of light in a waveguide provided on a substrate, these polarization states are generally referred to as the Transverse Electric (TE) mode and Transverse Magnetic (TM) mode. In the TM mode, the electric field of the light wave is oriented perpendicular to the substrate on which the waveguide sits, while in the TE mode the electric field is oriented parallel to the substrate.
Optical components, referred to as polarization rotators, can rotate the polarization of an optical signal, for example, from a TE mode to a TM mode. As generally understood, many rotators are made of a birefringent material, whereby the refractive index of such material along one polarization direction or birefringent axis is different than the refractive index in a different polarization direction or birefringent axis. The polarization of light propagating through such material can be resolved into different polarization components (e.g., a TE component and a TM component) and the refractive index difference causes these polarization components to travel at different speeds so that a phase difference develops between them. Accordingly, the net polarization of light traveling through the rotator, i.e., the vector sum of the polarization components, is rotated.
So-called “adiabatic rotators” have a geometry that introduces a gradual twist or rotation of the birefringent axes over the length of the rotator. As light propagates through the adiabatic rotator, the TE and TM polarization components will align with the birefringent axes of the rotator. If the twist is sufficiently gradual, the polarization states of the light will follow birefringent axes and rotate along with them. Adiabatic rotators may be wavelength independent and less fabrication sensitive and may not require that the polarization of light input to the rotator be aligned at a particular angle relative to birefringent axes.
Recently, photonic integrated circuits (PICs) have been proposed in which multiple optical components are provided on a semiconductor substrate. Adiabatic rotators, as noted above, gradually rotate the polarization of light supplied thereto, and are therefore relatively long. Accordingly, adiabatic rotators may not be adequately integrated onto a PIC having reduced dimensions.
Thus, there is a need for a polarization rotator having reduced size that can be integrated on a PIC.